In many instances such as retail or trade fairs it is desirable to present articles, e.g. fresh food, in an attractive way. With regard to illumination, this usually means that the colors of the articles should be enhanced.
Conventionally, compact high intensity discharge lamps, such as ultra high pressure sodium lamps (e.g. SDW-T lamps) or special fluorescent lamps are used for this purpose. In the case of light sources showing more continuous spectrum an additional filter is often used to obtain the required spectrum, leading however to low system efficacy. Additional drawbacks of these conventional light sources are relatively low efficacy and short lifetimes.
A light emitting diode (LED) based solution can in principle be used to overcome the above disadvantages. By combining light emitting diodes (LEDs) having different spectral output in the desired proportion, e.g. blue, green, amber and red, a total spectral output giving saturation of certain colors can be obtained. However, it is difficult to produce LEDs with a desired emission maximum. Other drawbacks of current LED based solutions are low efficiency and complexity of the system, as the use of differently colored LEDs leads to complex binning issues. Moreover, to maintain color point stability a complex control system is required, since particularly red LEDs exhibit strong changes in output spectra with current and temperature. As a result, the cost of the lamp is high.
In general lighting applications, some disadvantages of systems with LEDs of different colors can be overcome by using only blue LEDs and conversion of part of the blue light by a wavelength converting material (also referred to as a phosphor) to obtain white light output. However, a drawback of many blue light converting phosphors with regard to specialised illumination applications is that they generally exhibit a broad emission spectrum, and thus high saturation of colors cannot be achieved.
Furthermore, the known systems described above provide a predetermined light spectrum which may be suitable for enhancement of one or a few colors, at most. In retail environments, optimal illumination of all objects typically requires many different spectral compositions. For example, for illumination of fruit and vegetables green-enhanced (greenish) white light is desirable, and for cheese and meat yellow-enhanced and red-enhanced white light is desirable, respectively. Furthermore, for illumination of fish a cool white light is preferred, whereas for bread a warm white light gives the most visually appealing impression. Today there is no single system that can be used for optimal illumination of such differently colored articles.
US 2011/0176091 discloses a device having a variable color output. The device comprises an LED arranged in a light chamber, a luminescent element (phosphor), and an electrically variable scattering element, by which the color point and the correlated color temperature of the emitted light may be varied. The device may be adjusted to emit cool white light or warm white light. However, notwithstanding the disclosure of US 2011/0176091, there remains a need in the art for improved, color adjustable devices.